She followed him with her eyes, thinking: It’s as the Reverend Mother warned—too potent an adversary.

There are no more uses of "adversary" in the book.

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Then, again, our military training is in many respects superior to that of our adversaries.

Thucydides -- Pericles’s Funeral Oration

Finally, it was rumored, there would be complete information-sharing among all of these previously disconnected and even adversarial entities, and when they were coordinated, and once all the health data they’d collected was shared, most of this made possible through the Circle and more important, TruYou, viruses could be stopped at their sources, diseases would be tracked to their roots.